The EC reports a variety of error codes. Most of those, with the exception
of EC_RES_INVALID_VERSION, are converted to -EPROTO. As result, the actual
EC error code gets lost. Introduce cros_ec_map_error() to map EC error
codes to Linux error codes, and use it in cros_ec_cmd_xfer_status() to
report more meaningful errors to the caller. With this change, callers of
cros_ec_cmd_xfer_status() can implement a more distinguished action without
having to rely on the EC error code. At the same time, debugging is improved
in situations where the Linux error code is reported to userspace and/or in
the kernel log.

Cc: Gwendal Grignou <gwendal@chromium.org>
Cc: Yu-Hsuan Hsu <yuhsuan@chromium.org>
Cc: Prashant Malani <pmalani@chromium.org>
Cc: Brian Norris <briannorris@chromium.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>